Fainting Study: Just A Few Beers Can Knock You Off Your Feet

Even social drinking can sometimes cause healthy people to faint -- not due to drunkenness, but due to the way alcohol impairs the body's ability to tighten blood vessels.

It's been established that gravitational stresses exerted during the act of standing briefly cut blood flow to the brain -- one reason some people feel dizzy or lightheaded if they stand too quickly. Normally, blood vessels adjust by narrowing or constricting to keep blood pressure steady and prevent fainting.

But alcohol relaxes blood vessel walls, diminishing the ability of drinkers to adjust easily to the blood-pressure effects of standing after being seated. This change occurs even at mild intoxication levels of 105 milligrams per deciliter of blood -- just over the legal limit set by most states for drunk driving. It also produces a significant drop in blood pressure.

"We were a little surprised at the strength of the effect," said Mayo Clinic cardiologist Virend K. Somers, one of the authors of a study published in the journal Circulation.

The study examined the effects of mild intoxication -- the equivalent of three or fewer beers -- in 14 healthy young adults, average age 26. Blood pressure was measured in subjects before drinking, after drinking, while seated and as the subjects rose from a sitting to a standing position. In the study, systolic blood pressure -- the top number in a blood pressure reading -- dropped an average of 14 points while diastolic blood pressure fell by about eight points.

What the study suggests, Somers said, is that some people who already have blood-vessel changes may be vulnerable to fainting, after even small amounts of alcohol.

"People who are elderly or have diabetes or have heart disease or are on blood-pressure-lowering medication need to be more careful," he said. "We're not saying these people should not drink, but when they do drink, they need to be more careful about sudden changes in posture."